Background of the Invention US 5,827, 602 and US 5,965, 054 disclose non-aqueous ionic liquids for use as electrolytes that are based on salts composed of separate anion and cation moieties.

The cation moieties are selected from pyridinium, pyridazinium, pyrimidinium pyrazinium, imidazolium, pyrazolium, thiazolium, oxazolium, and triazolium ions.

US 6,090, 895 discloses crosslinked polymers having imide crosslinking groups and methods of crosslinking polymers to form imide crosslinking groups. These crosslinked polymers may be useful as polymer electrolyte membranes (PEM's) in fuel cells.

US 6,063, 522, US 4,505, 997, US 5,652, 072, US 5,072, 040 and US 5,514, 493 disclose electrolytes comprising imide functional groups.

US 5,463, 005 discloses perfluorinated monomers and polymers comprising sulfonyl and carbonyl imide groups for use as solid polymer electolytes.

M. Doyle, S. K. Choi and G. Proulx, J. Electrochem. Soc. , 147,34-37, discloses the use of ethyl methyl imidazolium (EMI) triflate, a low melting ionic liquid composed of separate anion and cation moieties, together with a known sulfonate fluoropolymer (Nafion) to give a fuel cell membrane material for use at elevated temperatures.

Summary of the Invention Briefly, the present invention provides a zwitterionic imide compound according to the formula: Rl-S02-N--SO2-R2+, where R1 and R2+ are any suitable

groups. Typically Rl is a highly fluorinated alkane and R2+ is a heteroatomic aromatic group having an cationic nitrogen, such as: pyridiniumyl, pyridaziniumyl, pyrimidiniumyl, pyraziniumyl, imidazoliumyl, pyrazoliumyl, thiazoliumyl, oxazoliumyl, or triazoliumyl. Typically the zwitterionic imide has a melting point of less than 100 °C. Typically the zwitterionic imide compound has a solubility in water of less than 5% by weight.

In another aspect, the present invention provides a zwitterionic liquid having a melting point of less than 100 °C and typically having a solubility in water of less than 5% by weight.

In another aspect, the present invention provides an electrochemical device comprising the zwitterionic liquid or the zwitterionic imide according to the present invention, such as a battery or a fuel cell.

What has not been described in the art, and is provided by the present invention, is an ionic liquid having a melting point of less than 100 °C that is a zwitterionic compound, particularly a zwitterionic imide compound.

In this application: "highly halogenated", means containing halogen in an amount of 40 wt% or more, but typically 50 wt% or more, and more typically 60 wt% or more; and "highly fluorinated", means containing fluorine in an amount of 40 wt% or more, but typically 50 wt% or more, and more typically 60 wt% or more; and "substituted"means, for a chemical species, substituted by conventional substituents which do not interfere with the desired product or process, e. g., substituents can be alkyl, alkoxy, aryl, phenyl, halo (F, Cl, Br, l), cyano, nitro, etc.

It is an advantage of the present invention to provide an electrolyte for use in electrochemical devices such as batteries and fuel cells at temperatures in excess of 80 °C. The zwitterionic liquids can be more stable in use than conventional (non- zwitterionic) ionic liquids, since they do not form a neutral, volatile, water soluble species upon protonation of the anion.

Detailed Description of Preferred Embodiments The present invention provides zwitterionic imide compounds according to the formula: R1-SO2-N--SO2-R2+, where R1 and R2+ are any suitable groups. Typically R1 is a highly fluorinated alkane and R2+ contains a heteroatomic aromatic group having an cationic nitrogen, such as: pyridiniumyl, pyridaziniumyl, pyrimidiniumyl, pyraziniumyl, imidazoliumyl, pyrazoliumyl, thiazoliumyl, oxazoliumyl, or triazoliumyl.

The present invention additionally provides a zwitterionic liquid having a melting point of less than 100 °C and typically having a solubility in water of less than 5% by weight, which may be a zwitterionic imide compound according to the present invention.

Electrochemical devices such as batteries and fuel cells employ electrolyte materials to conduct ions participating in electrochemical reactions. Common aqueous based electrolytes are not suitable for use at high temperatures in excess of 100 °C, since the water component can evaporate. Drying of the electrolyte can become a problem at temperatures of 80 °C or greater.

Polymer electrolyte membranes (PEM's) may be used in electrochemical devices, particularly in fuel cells. PEM's are composed of a polymer backbone, typically fluorinated, having pendant acidic groups such as sulfonic acid groups.

Examples include NafionX) (DuPont Chemicals, Wilmington DE) and FlemionTm (Asahi Glass Co. Ltd. , Tokyo, Japan). Non-fluorinated PEM's include sulfonate polysulfone and sulfonate PEEK. In a fuel cell, the PEM conducts H+ ions (protons) while forming a barrier to the passage of reactant gasses (H2 and °2) The PEM is electrically non-conductive. However, membranes of these polymers must contain a liquid electrolyte, typically water, to function electrolytically. Even though water is produced during the electrochemical reactions carried out in a hydrogen fuel cell, drying of the PEM can become a problem at temperatures of 80 °C or greater and the cell cannot operate with a water electrolyte at temperatures in excess of 100 °C.

Conventional (non-zwitterionic) ionic liquids can be of limited utility in extreme environments where the anion can become protonated. The resulting neutral acid may be volatile and water soluble. This is a particular concern for fuel cell use,

since a volatile, water soluble species will be washed out of the cell, rendering it inactive.

The zwitterionic liquids according to the present invention can be used as electrolytes in electrochemical devices such as batteries and fuel cells. The zwitterionic liquids may be used neat or with a solvent, diluent or additive. Additives may be used to alter melting point, viscosity, ionic conductivity, and the like. Further, the zwitterionic liquids according to the present invention can be absorbed into polymer electrolyte membranes for use in electrochemical devices, especially for use in fuel cells. The zwitterionic liquids may be substituted for water in a typical polymer electrolyte membrane by any suitable method. In one method, the zwitterionic liquid is applied to the membrane and/or the membrane is immersed in the zwitterionic liquid, with these steps being repeated as necessary.

The zwitterionic liquids according to the present invention typically have a melting point of less than 100 °C, more typically less than 80 °C, more typically less than 60 °C, and most typically less than 40 °C. The zwitterionic liquids according to the present invention typically have a solubility in water of less than 5% by weight, more typically less than 1% by weight, and most typically less than 0. 1% by weight.

In one embodiment, the zwitterionic liquid according to the present invention is a zwitterionic imide compound according to formula (I) : Rl-S02-N--S02-R2+ (1) where R1 and R2+ are any suitable groups.

Rl may be any group which does not interfere with the desired characteristics of the compound. Ri may be selected from straight-chain, branched, cyclic and aromatic groups, including saturated and unsaturated groups, including heteroatomic groups, and including any of the above which are substituted. Typically Rl is a highly halogenated alkane, more typically a highly fluorinated alkane. More typically Rl is a perhalogenated alkane, and most typically a perfluorinated alkane. Typically Rl contains between 1 and 20 carbon atoms, more typically between 1 and 8 carbon atoms, and most typically between 1 and 4 carbon atoms.

R2+ may be any cationic group which does not interfere with the desired characteristics of the compound. R2+ may be selected from straight-chain, branched, cyclic and aromatic groups, including saturated and unsaturated groups, including heteroatomic groups, and including any of the above which are substituted. Typically, R2+ is an aromatic group, more typically an aromatic heterocyclic group. More typically, R2+ is a cationic heteroatomic aromatic group including nitrogen. More typically, R2+ is selected from the following: pyridiniuinyl N I R pyridaziniumyl N ru N I R N pyrimidiniumyl N R IN pyraziniumyl N I R i w imidazoliumyl R/N\ R N pyrazoliumyl/\N I R s thiazoliumyl

1-i 0"N oxazoliumyl N- triazoliumyl NW where R groups are independently selected from H, alkyl, haloalkyl, fluoroalkyl, or any group which does not interfere with the desired characteristics of the compound. R may be selected from straight-chain, branched, cyclic and aromatic groups, including saturated and unsaturated groups, including heteroatomic groups, and including any of the preceding which are substituted. Typically R contains between 0 and 20 carbon atoms, more typically between 0 and 8 carbon atoms, and most typically between 0 and 4 carbon atoms. The above-listed R2+ groups may be additionally substituted at any available carbon. Substituents may join with each other or with R groups to form additional aromatic or non-aromatic rings. Alternately, the open valence of the R2+ group, i. e. , the imide attachment point, may be on the R group or other substituent, rather than on the aromatic ring as depicted above. Typically, the zwitterionic imide contains an imide bound directly to an aromatic group. The imide may be bound to a carbon or to a nitrogen of the aromatic group.

Alternately, R2+ may be a quaternary ammonium group. Examples of such quaternary ammonium groups include tetraalkyl ammonium cations. Where R2+ is a tetraalkyl ammonium group, alkyl substituents of the tetraalkyl ammonium group typically contain 1 to 12 carbons, more typically 1 to 8 carbons, and more typically 1 to 4 carbons. Alkyl substituents of the tetraalkyl ammonium group may be substituted or may link to form ring structures containing the ammonium nitrogen. Alkyl substituents of the tetraalkyl ammonium group may be substituted with polymerizable functional groups. The imide may be bound to a carbon or to the nitrogen of the ammonium group.

The zwitterionic imide according to the present invention can be made by any suitable method. Known imide syntheses may be performed using the appropriate starting materials containing the desired Rl and R2+ groups, including syntheses

described in: US 6,063, 522, US 4,505, 997, US 5,652, 072, US 5,072, 040, US 5,514, 493 and US 5,463, 005. Useful synthetic routes may include: Rl-S02-NH2 + Cl-S02-R2+ < Rl-S02-N--S02-R2+ (Il) Rl-S02-Cl + H2N-S02-R2+ < Rl-S02-N--S02-R2+ It will be recognized that reactions (1) and (II) are typically carried out in the presence of additional base such as (Et) 3N.

If desired, the cationic group of R2+ may be converted with a protecting group during synthesis and unprotected after synthesis.

This invention is useful as an electrolyte in devices such as batteries and fuel cells, particularly high-temperature devices for operation at temperatures in excess of 100 °C.

Various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the scope and principles of this invention, and it should be understood that this invention is not to be unduly limited to the illustrative embodiments set forth hereinabove.